On the trail of a killer grizzly bear.

The Wapiti sow’s first violent encounter with a human came on the morning of Wednesday, July 6, with the park still bustling after the holiday weekend. Based on all the evidence gathered by the park rangers in the weeks that followed, the incident began when the sow and her two cubs were grazing and digging in an open meadow below the Wapiti trail. Though the Trail is fairly easy to access, it brings you to the backcountry, and some remote hot springs that few Yellowstone visitors ever get to see.

It's likely the bear didn't feel threatened at first, when the scent of a pair of human hikers, 58-year-old Brian Matayoshi and his wife Marylyn, blew in her direction. The Matayoshis took pictures of the three bears from their perch on the trail above and then kept hiking, with no reason to think they would ever see those animals again.

Less than 30 minutes later, the Matayoshis came face-to-face with the sow on the trail. They started hiking away from her immediately, looking back to see what she was doing. Then Marylyn saw the 250-pound sow lift up her head and start to chase after them. Brian cried out, "Run!"

Bear biologists recommend two strategies for dealing with a grizzly approaching from close range—stand your ground or slowly back away. If the Matayoshis had done either of those things, it's possible the Wapiti sow would have wandered off, or maybe bluff-charged to make sure they weren’t going to hurt her babies. But to see the pair running made her want to chase them even more.

The Matayoshis hadn't gotten far down the path when the Wapiti sow caught up with them. She reached Brian first, knocking him down and raking his scalp. Then she started biting and scratching his right thigh, and punctured his femoral artery. It’s unclear how long the assault continued—Marylyn couldn’t watch. When she looked up from her hiding place behind a downed log, she saw the Wapiti sow standing over her husband. The two made eye contact—a gesture that a grizzly might take as an attempt to intimidate—and then the bear lunged toward her and ripped at her backpack with its claws, before cutting off the attack and running into the woods.

Marylyn Matayoshi doesn’t know how long she waited, motionless, before going over to her husband. She tied her jacket around his thigh in an effort to stanch the bleeding, but it was too late. Brian exhaled his last breath just minutes after the sow and cubs had fled the scene—the first death by grizzly mauling in Yellowstone since 1986.

The same grizzly CSI team that would investigate John Wallace’s death several weeks later did a thorough sweep of the Matayoshi crime scene. The evidence in this case was far easier to interpret. For one, investigators had an eyewitness. Marylyn had seen the attacking bear, both on the trail and in the meadow, and she'd seen her two cubs, one with a distinctive blond face. That alone was enough to make an identification. The Wapiti sow—the one James Yule had filmed in June—was the only bear rangers had seen in the Hayden Valley that matched the description.

It would have been possible to trap the Wapiti sow at that point and euthanize her if necessary. But Servheen and the other bear managers decided to leave the Wapiti sow alone. Her behavior, they thought, was natural considering the circumstances. She attacked the Matayoshis because she was defending her cubs. If one of those bites hadn’t landed on Brian's femoral artery, the rangers thought, he might have survived the mauling as most others do. (There have been more than 125 grizzly bear attacks in national parks since 1900, and fewer than 30 fatalities.) The tragic outcome of the case seemed to be a matter of some very bad luck.

Because bear managers were so certain that the Wapiti sow had been acting in natural self-defense, they didn’t even put a radio collar on her. When they looked for her from a helicopter over Hayden Valley that day, she was 2 miles from the scene of the attack, walking around and foraging with her cubs. They knew they were looking at the bear that killed Brian Matayoshi, but they let her be.

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